Nonwoven webs and their manufacture from melt-processable thermoplastic polymers has been the subject of extensive development resulting in a wide variety of materials for numerous commercial applications. Nonwoven webs formed from a spunbond process consist of a sheet of overlapped and entangled filaments or fibers of melt-processable thermoplastic polymers. A spunbond process generally involves extruding a dense curtain of semi-solid filaments from a spinneret of a spin pack. The descending curtain of filaments is cooled by a cross flow of cooling air and the individual filaments are attenuated or drawn by a filament drawing device or aspirator. Spunbond filaments are generally lengthwise continuous and have average diameters in the range of about 10 to 20 microns. Filaments discharged from the drawing device are collected as a sheet of entangled loops on a collector, such as a forming belt or a forming drum, and are deposited as a continuous length nonwoven web.
Various different types of conventional drawing devices are available for use in meltspinning apparatus. Generally, a drawing device receives the curtain of filaments descending from the spinneret in a slotted passageway and directs a high-velocity stream of process air at the filaments from one or more venturis or air jets exhausting into the passageway. Each air stream is oriented substantially tangential to the filament length and exerts a drawing force on the filaments that increases the filament velocity. The drawing force attenuates the filaments in the space between the spinneret and the drawing device inlet and in the space between the drawing device and the collector. In addition, the polymer chains constituting the filaments may be oriented if the filament velocity or spinning speed is sufficiently high.
Certain characteristics of the high-velocity stream of process air used to attenuate the filaments are believed to degrade the quality of the collected nonwoven web. In one aspect, the high-velocity stream of process air exiting the venturis creates lateral vortices that travel down the confronting planar surfaces defining the slotted passageway and eventually exit the passageway outlet along with the filaments and high-velocity process air. The interaction of the lateral vortices with the descending filaments and the high-velocity of the stream of process air causes unpredictable variations in the looping of the filaments. As a result, localized areas of relatively low web density and relatively high web density occur that reduce the long range uniformity of the collected nonwoven web. This loss of uniformity may be undesirable for those end products intended to be fluid impervious, as the low-density areas define unacceptable leakage paths that defeat use as a barrier material.
The high-velocity process air aspirates secondary air from the environment adjacent the outlet, which mixes with the process air and filaments at the end and side boundaries of the outlet from the drawing device. The mixing causes the airborne filaments to oscillate in a chaotic and random manner in the flight path from the outlet of the drawing device to the collection device. The randomized movement of the airborne filaments decreases the integrity of the nonwoven web due to variations in coverage. The aspirated secondary air at the end boundaries of the outlet also produces inwardly-directed currents of secondary air that cause filaments exiting adjacent to the end boundaries to move inwardly as they travel toward the collection device, which increases the local filament density adjacent to the end boundaries. As a result, the opposite peripheral margins of the nonwoven web have an increased basis weight.
A conventional technique for decreasing the randomness and chaotic character of the paths traced by filaments during their descent to the collector is to provide the drawing device with rows of thin fingers or guide fins upstream of the outlet. Conventional guide fins are formed of bent strips of thin sheet metal arranged into two rows extending in the cross-machine direction, which are separated by an open space or tunnel. Guide fins in the upstream row are inclined and those in the downstream row are oriented vertically. Adjacent pairs of guide fins in each row are separated by a small gap. The guide fins in the downstream row are arranged to be offset by one-half of the row pitch from the guide fins in the upstream row so that the upstream row is not covered.
Nevertheless, the rows of guide fins fail to prevent the difficulties associated with the mixing of aspirated secondary air and the high-velocity process air exiting the drawing device and introduce additional artifacts into the structure of the nonwoven web. Secondary air is aspirated through the gaps between adjacent guide fins in each row and flows through the space between the two rows. The aspirated air flowing through the gaps between the guide fins toward the filaments causes filaments being guided by the upstream row to shift laterally (i.e., in the cross-machine direction) so that the resultant nonwoven web has alternating low-density and high-density stripes spaced across the width of the web with the periodicity of the guide fin pitch. The striping reduces the integrity of the nonwoven web and causes undesirable formation variations.
Raising the drawing device away from the collection device reduces the striping and increases filament entanglement and web integrity. However, as the distance is increased between the drawing device outlet and the collection device, chaotic movement of the filaments increases the loop size of the collected filaments and bundling or twisting. Web quality is reduced by the occurrence of random localized areas of relatively low web density and areas of relatively high web density.
Conventional guide fins cannot eliminate the lateral vortices from the high-velocity air exiting the drawing device, which further increases the randomness of, and lack of control over, the trajectories of the descending filaments. Because the guide fins are formed from bent sheet metal, they lack robustness and are easily bent out of position by accidental contact.
A need exists, therefore, to improve the stability and the guidance of airborne filaments descending from the drawing device to the collector.